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Slowing warming Plants could grow larger and create a cooling effect in a world with twice as much carbon dioxide, but could not halt or reverse climate change, according to a NASA study.

One of the main mysteries scientists face with climate change is how to project it over time, particularly how to account for Earth's reaction to warmer temperatures, a phenomenon known as 'feedback'.

It has long been known that plants - which use carbon dioxide, sunlight and water to grow through the process of photosynthesis - are able to adapt to higher carbon dioxide levels by using nutrients more efficiently and growing bigger leaves.

"The process is called 'down-regulation.' This more efficient use of water and nutrients has been observed in experimental studies and can ultimately lead to increased leaf growth," says NASA in a statement.

But just what effect increased plant growth would have on global warming has been difficult to predict.

NASA's new computer modeling effort, described in the latest issue of the journal Geophysical Research Letters, calculated the plants' cooling effect to be -0.3°C globally.

But that number falls well short of the temperature increase of 2°C to 4.5°C that is the standard basis for many global warming modelling simulations.

"This feedback slows but does not alleviate the projected warming," says Lahouari Bounoua, lead author of the paper and expert at the Goddard Space Flight Center.

Bounoua's model found that warming was on the lower scale of the widely accepted range when carbon dioxide was doubled, projecting warming of 1.94°C globally, not including the 0.3°C drop for feedback.

According to NASA, previous climate models have not included the larger leaf growth that would come from 'down-regulation', and have projected little to no cooling from vegetation growth.

"This is what is completely new," says Bounoua, referring to how the model was adapted to include changed leaf growth.

"What we did is improve plants' physiological response in the model by including down-regulation. The end result is a stronger feedback than previously thought."

The latest research shows "how, over time, scientists will create more sophisticated models that will chip away at the uncertainty range of climate change and allow more accurate projections of future climate," says NASA.

"As we learn more about how these systems react, we can learn more about how the climate will change," says Hall.

"Each year we get better and better. It's important to get these things right just as it's important to get the track of a hurricane right. We've got to get these models right, and improve our projections, so we'll know where to most effectively concentrate mitigation efforts."